For determining parameters, for example a mass flow (e.g. mass flow rate), a density, a viscosity, etc., of media, for example liquids and/or gases, flowing in a pipeline, often inline measuring devices are used, especially inline measuring devices embodied as Coriolis mass flow meters, which by means of a measuring transducer of vibration-type and a control and evaluation electronics connected thereto, induce forces, for example Coriolis forces, in the flowing medium and produce, derived from these forces, a measurement signal representing the at least one parameter. Such inline measuring devices with a measuring transducer of vibration type have been known for a long time and have established themselves well in industrial application. Thus, for example in EP-A 317 340, U.S. Pat. Nos. 5,398,554, 5,476,013, 5,531,126, 5,691,485, 5,705,754, 5,796,012, 5,945,609, 5,979,246, 6,006,609, 6,397,685, 6,691,583, 6,840,109, WO-A 99 51 946, WO-A 99 40 394 or WO-A 00 14 485, Coriolis mass-flow meters are described, each with a measuring transducer of vibration-type. Each of the disclosed measuring transducers includes a single, straight measuring tube, which conveys the medium and vibrates during operation. Such measuring tube communicates with the pipeline via an inlet tube piece at its inlet end and an outlet tube piece at its outlet end. Each of the disclosed measuring transducers also includes: An exciter mechanism, which causes the measuring tube during operation, by means of at least one electromechanical, especially electrodynamic, oscillation exciter acting thereon, to oscillate with bending oscillations in a tube plane; and a sensor arrangement having oscillation sensors, especially electrodynamic oscillation sensors, for the at least point-wise registration of oscillations toward the inlet end and toward the outlet end of the measuring tube and for producing electrical sensor signals influenced by the mass flow. Additionally, each of the disclosed measuring transducers has a transducer housing encasing the measuring tube with counteroscillator coupled thereto and encasing also the provided exciter mechanism and sensor arrangement, especially a transducer housing affixed directly to the inlet tube piece and to the outlet tube piece. Besides the oscillation sensors provided for registering vibrations of the measuring tube, the measuring transducer can, as proposed also in, among others, EP-A 831 306, U.S. Pat. Nos. 7,040,179, 5,736,653, 5,381,697 or WO-A 01/02 816, have yet other sensors arranged on the inner part and serving especially for registering perhaps secondary measured variables, such as e.g. temperature, acceleration, strain, stress, etc.
As is known, straight measuring tubes, when excited to bending oscillations of a first eigenoscillation form (the so-called driving-mode, or also, wanted-mode), effect Coriolis forces in the medium flowing through the measuring tube. These forces, in turn, lead to a superimposing, on the excited bending oscillations, of coplanar bending oscillations of a second form of eigenoscillation of higher and/or lower order (the so-called Coriolis mode), such that oscillations registered on the inlet and outlet sides of the measuring tube exhibit also a measurable phase difference dependent on the mass flow. Usually the measuring tubes of such measuring transducers, especially those used in Coriolis mass-flow meters, are excited in the driving-mode to an instantaneous resonance frequency of the first eigenoscillation form, especially at oscillation amplitude controlled to be constant. Since this resonance frequency is especially also dependent on the instantaneous density of the medium, it is possible also by means of Coriolis mass-flow meters common in the market to measure, besides mass flow, also the density of flowing media.
An advantage of straight measuring tubes is that; for example, they can be emptied with a high degree of certainty completely in practically any installation orientation. Especially is this also true after a cleaning process performed inline. Additionally, such measuring tubes are, in comparison e.g. to omega-shaped or helically-shaped measuring tubes, essentially easier and accordingly more cost-favorably manufacturable. A further advantage of a straight measuring tube vibrating in the above described manner is, in comparison to bent measuring tubes, also to be seen e.g. in the fact that, during measurement operations via the measuring tube, practically no torsional oscillations are evoked in the connected pipeline. On the other hand, a significant problem of the above-described measuring transducers lies in the fact that, because of the alternating lateral deflections of the vibrating, single measuring tube, oscillating transverse forces of the same frequency can be caused to act on the pipeline. To this point in time, these transverse forces have only been able to be compensated to a very limited extent and only with a very high technical effort.
For improving the dynamic balance of the measuring transducer, especially for reducing transverse forces caused by the vibrating, single measuring tube acting at its inlet and outlet ends on the pipeline, the measuring transducers disclosed in EP-A 317 340, U.S. Pat. Nos. 5,398,554, 5,531,126, 5,691,485, 5,796,012, 5,979,246, 6,006,609, 6,397,685, 6,691,583, 6,840,109 or WO-A 00 14 485 include in each case a counteroscillator embodied as one or more pieces and affixed to the measuring tube on the inlet end, accompanied by the formation of a first coupling zone, and affixed to the measuring tube on the outlet end, accompanied by the formation of a second coupling zone. Such counteroscillators, which are implemented in the form of a beam or especially in tubular form or as a body pendulum aligned with the measuring tube, oscillate during operation out of phase with the measuring tube, especially with opposite phase, whereby the effect of the lateral transverse forces and/or transverse impulses brought about in each case by the measuring tube and the counteroscillator on the pipeline can be minimized and in some cases also completely suppressed.
In the case of commonly marketed measuring transducers having a single measuring tube and a counteroscillator coupled thereto, the oscillation exciter of the exciter mechanism is formed by means of: At least one coil, which is usually affixed to the counteroscillator, and has current flowing through it, at least at times, and a magnetic field passing through it, at least at times; and an armature affixed to the measuring tube and interacting with the at least one coil. In the case of most measuring transducers of the described kind, the oscillation sensors of the sensor arrangement are constructed on the same principle as the aforementioned oscillation exciter. Accordingly, also the oscillation sensors of such a sensor arrangement are, most often, each formed by means of: At least one coil, which is usually affixed to the counteroscillator, and has current flowing through it, at least at times, and a magnetic field passing through it, at least at times; and an armature affixed to the measuring tube and interacting with the at least one coil. Each of the aforementioned coils is, additionally, connected with the mentioned operating and evaluating electronics of the inline measuring device by means of at least one pair of electrical connection lines. The connection lines are usually guided on the shortest possible path from the coils via the counteroscillator to the transducer housing.
Measuring transducers of the described kind having a single measuring tube and counteroscillator have proven themselves, especially in the case of those applications wherein the medium to be measured has an essentially constant density or a density which changes to only a very slight degree, thus, for those applications in which a net force acting on the attached pipeline, resulting from the transverse forces produced by the measuring tube and the counterforces produced by the counteroscillator, can initially be set, without more, assuredly to zero. In contrast, those measuring transducers, especially those disclosed in U.S. Pat. Nos. 5,531,126 or 5,969,265, in the case of applications with media having densities fluctuating over wide ranges, especially in the case of different media following one after the other, and even when to only a slight degree, exhibit practically the same disadvantage as measuring transducers without counteroscillators, since the above-mentioned net resultant forces are also dependent on the density of the medium and consequently can be different from zero to a considerable degree. Stated differently, also the inner part of the measuring transducer formed by at least the measuring tube and the counteroscillator is globally deflected during operation out of an assigned static rest position, due to density dependent imbalances and transverse forces associated therewith.
A possibility for reducing density dependent, transverse forces is described e.g. in U.S. Pat. Nos. 5,287,754, 5,705,754, 5,796,010 or 6,948,379. In the case of the measuring transducers shown there, the more middle, or high, frequency, oscillatory, transverse forces produced on the part of the vibrating, single measuring tube are kept away from the pipeline by means of an, in comparison to the measuring tube, very heavy counteroscillator, and, as required, a relatively soft coupling of the measuring tube to the pipeline, thus, in practical terms, by means of a mechanical low pass filter. A great disadvantage of such a measuring transducer is, among other things, however, that the counteroscillator mass required for achieving a sufficiently robust damping increases more than proportionately with the nominal diameter of the measuring tube. On the other hand, when using such a massive counteroscillator, one must assure that a minimum eigenfrequency of the measuring transducer (which becomes ever lower with increasing mass) still lies far from the likewise very low eigenfrequencies of the attached pipeline. Different, farther-reaching possibilities for reduction of the density dependent, transverse forces are proposed e.g. in U.S. Pat. Nos. 5,979,246, 6,397,685, 6,691,583, 6,840,109, WO-A 99 40 394 or WO-A 00 14 485. In the case of the disclosed compensation mechanisms presented there, of essential concern is the expanding of a bandwidth, within which counteroscillator and offset sections are effective, by providing a suitable interaction of the individual components of the inner parts of the measuring transducers. In particular, in U.S. Pat. No. 6,397,685, a measuring transducer of the aforementioned kind is disclosed, wherein a first balancing mass is provided as a mass balancing measure for the exciting oscillation and is connected with the counteroscillator in the longitudinal-axis-perpendicular, central plane of the counteroscillator (which is embodied as a compensation cylinder). Then, second and third balancing masses are provided as a mass balancing measure for the Coriolis oscillation. The second and third balancing masses are embodied as end regions of the counteroscillator. In this manner, it is to be achieved that the inner part composed of the measuring tube and the compensation cylinder is at least largely balanced with respect to mass both for the exciting oscillations of the measuring tube as well as also for the Coriolis oscillations of the Coriolis measuring tube. WO-A 00 14 485 also describes a measuring transducer of vibration-type for a medium flowing in a pipeline. In this case, provided are: An inlet end, first cantilever, which is coupled with the measuring tube in the region of a third coupling zone lying between the first and second coupling zones and which has a center of mass lying in the region of the measuring tube; and an outlet end, second cantilever, which is coupled with the measuring tube in the region of a fourth coupling zone lying between the first and second coupling zones and which has a center of mass lying in the region of the measuring tube. Each of the two cantilevers is provided for executing balancing oscillations, which are so developed that the transverse impulses are compensated, and, consequently, a center of mass of an inner part formed of measuring tube, exciter mechanism, sensor arrangement and the two cantilevers is held locationally fixed. Furthermore, WO-A 99 40 394 describes a measuring transducer of the aforementioned kind in which a first cantilever serving for producing counterforces acting against the transverse forces at the inlet ends, as well as a second cantilever serving for producing counterforces acting against the transverse forces on the outlet end are provided. In such case, the first cantilever is affixed both to the measuring tube in the region of the first coupling zone and also to the transducer housing at the inlet end, and the second cantilever is affixed both to the measuring tube in the region of the second coupling zone, as well as also to the transducer housing on the outlet end, such that the counterforces are so developed that the measuring tube is kept fixed in an assigned, static rest position, despite the produced transverse forces. Finally, in U.S. Pat. Nos. 6,691,583 and 6,840,109, measuring transducers are in each case disclosed, wherein, in each case, a first cantilever fixed in the region of the first coupling zone essentially rigidly to the measuring tube, counteroscillator and inlet tube piece and a second cantilever fixed in the region of the second coupling zone essentially rigidly to the measuring tube, counteroscillator, and outlet tube piece are provided. The two cantilevers, especially ones arranged symmetrically about the middle of the measuring tube, serve here for producing in the inlet and outlet tube pieces bending moments dynamically, when the vibrating measuring tube together with the counteroscillator and, as a result, also the two coupling zones are shifted laterally from their respectively assigned, static, rest positions, with the bending moments being so developed that, in the deforming inlet tube piece and in the deforming outlet tube piece, impulses are produced, which are directed counter to the transverse impulses produced in the vibrating measuring tube. The two cantilevers are so embodied and so arranged for this purpose in the measuring transducer that a center of mass of the first cantilever lying in the region of the inlet tube piece and a center of mass of the second cantilever lying in the region of the outlet tube piece both remain essentially locationally fixed in a static rest position despite the fact that the measuring tube has been shifted laterally out of its assigned static rest position. The basic principal of this compensation mechanism is to transform lateral displacement movements of the vibrating measuring tube, which would otherwise act in a disturbing manner on the measurements and/or on the connected pipeline and which are superimposed on its primary deformations effecting the measurement effects, into counter deformations of the inlet and outlet tube pieces acting in a dynamically balancing manner in the measuring transducer, in order to largely eliminate the lateral deflection movements. By a suitable tuning of the inner part, the deformations of the inlet and outlet tube pieces can be so developed that the transverse impulses largely compensate one another, independently of the instantaneous oscillation amplitudes and/or frequencies of the measuring tube. In corresponding manner, it is thus possible also essentially to compensate the transverse forces produced by the vibrating measuring tube by means of transverse forces produced by the deforming inlet tube piece and the deforming outlet tube piece.
Investigations on measuring transducers of the described kind have, however, shown, that, despite the inner part, as discussed above, being almost perfectly mechanically balanceable, even in the case of fluctuating density, still considerable disturbances can arise in the oscillation measurement signals. Especially, it has been found, firstly, that these disturbances are not only of equal frequency to the oscillations of the measuring tube, but also that these disturbances unfortunately also present themselves directly in the phase difference essential for the mass flow measurement and, consequently, can lead to a not inconsiderable corruption of the measurement result. Additionally, these disturbances can arise in rather non-reproducible, and, as a result, without extra effort, unpredictable, manner. Accompanying this, a subsequent, for example even algorithmic, compensation of these disturbances of the measurement signals is practically impossible. More extensive investigations have additionally shown, that the disturbances of the aforementioned kind are caused, at least mediately, by the above-mentioned connection lines.
Further, it has been found, that, especially in the sections of the connection lines, which extend practically freely suspended between the inner part and the transducer housing, interference voltages, or currents, can be induced directly in the connection lines, when the inner part is oscillating. These interference voltages, or currents, are induced due to movements of the individual connection lines relative to one another, leading to changes with time of the capacitive and inductive line- and/or stray-impedances.
Moreover, it has been discovered, that, in the case of conventional routing of the connection lines, for example, along a section of the counteroscillator, then over a freely suspended segment between two tie-down points for each of the connection lines, to the transducer housing, alone due to the damping effect of the relatively thin, moved wires and insulations of the lines, there will be imposed on each of the two sensor signals an additional phase-shift, and, indeed, in a manner changing the phase difference; this happens, in particular, also despite effective suppression of fluctuating-density-caused, lateral displacements of the inner part, formed by means of the measuring tube and counteroscillator, relative to the transducer housing. In other words, the connection line influences the zero-point of the measuring transducer to such an extent that, even in the case where the measuring tube of the measuring transducer is not flowed through by medium, a mass flow different from zero would be, erroneously, detected. Making the situation even more difficult, these zero-point displacements caused by the connection lines depend in quite significant measure on the operating-temperature and/or -duration of the measuring transducer.
In connection with the disturbances caused by the connection lines, especially the aforementioned freely suspended segment has turned out to be an interference-causing, and, thus, for accuracy of measurement, neuralgic region, such being true, surprisingly, even for inner parts with a relatively massive and heavy, counteroscillator. Thus, in the aforementioned region, inner part and transducer housing are mechanically coupled together; and while this mechanical coupling is perhaps weak, nevertheless, for the aforementioned null-point stability, it is not insignificant. By the relative movement of the two tie-down points in each case intercepting the freely suspended line sections, the thereby necessarily deformed and/or moved line sections bring about their damping action, in such case, unfortunately in a manner such that the phase difference between the two sensor signals is changed. It was possible, in such case, it is true, to determine that, by joining the connection lines to form a cable harness, along with placing of the one of the aforementioned tie-down points near an oscillation node of the above-mentioned Coriolis mode, thus practically at the center of the counter oscillator, a certain lessening of the disturbances can be achieved. However, it was, unfortunately, also determined, that the aforementioned zero-point error again is of considerable degree at just a very small departure from the perfectly central position, for instance even at the order of magnitude of manufacturing and/or mounting tolerances, and, associated therewith, at only a small eccentricity of the effective damping force or also a small asymmetry of a damping force burden relative to the mentioned oscillation node; this is true even more so in the case of a counteroscillator oscillating with considerable amplitude.
Finally, the aforementioned freely suspended segment is, because of the high number of cycles of the inner part over the total operating time, also a mechanically highly loaded part of the connection lines, so that a corresponding selection of fatigue-resistant materials for the line wires and insulation, as well as appropriate material thicknesses, are required. Accordingly, the connection lines can, both for electrical reasons and for reasons of mechanical strength, not be kept arbitrarily thin, and, along therewith, cannot be made mechanically insignificant as regards the zero-point error.